This investigation defines mechanisms of visual transduction in retinal cones. Cones are photoreceptor cells primarily responsible for the color discrimination, contrast sensitivity, and resolution in time and space of daylight vision. The sequence of molecular events coupling photon absorption in visual pigment to membrane excitation, phototransduction, is unknown. Methods developed in this laboratory directly measure the light-modulated voltage clamped current of single isolated cones. Clamp speed and spatial uniformity are adequate to resolve kinetics of the flash-evoked photocurrent. Access via gigaseal pipettes enables ready transport of experimental solutes into the cell. Simultaneous suction electrode and whole-cell gigaseal voltage clamp recording measures photocurrent before, during, and after intracellular penetration. Major gains in understanding transduction in retinal rods have recently been made through direct physiologic demonstration of cyclic GMP (cGMP) control of the light-sensitive conductance and by in vitro biochemical characterization of the light-activated phosphodiesterase (PDE) cascade. The hypothesis that a similar cGMP-PDE cascade mediates transduction in cones will explore selected steps in the excitatory sequence. The effects of exogenous substrates, inhibitors, and activators each appropriate to particular steps of the cascade will be examined. The hypothesis that differing reaction rates of the PDE cascade produce the gain and bandwidth properties distinguishing cone from rod transduction will be tested by derivation of reaction rates from kinetic properties of the light response. The hypothesis that the light adaptation properties of cones can be explained by a feedback step mediated by the flow of calcium ions through the light-modulated conductance acting internally on the activation of PDE will be tested by examination of the intracellular effects of calcium chelators.